How to build a High Performance Shovelhead Engine

The following information for building a high performance
Shovelhead engine was discovered in the archives of a Harley-Davidson
dealership. The list of modifications has been attributed to the HD Engineering
Group responsible for XR-750 development in the late 70's or early 80's. While
it is probably not possible to validate this claim, some of the ideas
presented in the modification list are very interesting. Many of the recommend
modifications are closely aligned with performance work being done in the
90's, not 1970's technology as practiced by many Shovelhead mechanics.

This does lead us to the conclusion that the originator of
this document was familiar with advances in air flow technology as practiced
by engine developers like Colin Chapman (Lotus Formula 1 racing, Cosworth
Engineering) in the 60's. It is an interesting footnote that the concept of
high velocity port design did not make it to mainstream Harley-Davidson
performance engine modifications until the mid to late 90's. This Shovelhead
modification document distinctly shows "D" shaped ports, optimized
for high velocity over the more popular "bigger is better"
philosophy that was used then, and unfortunately is still used today.

At least one Shovel engine has been built using many of the
ideas present here, with excellent results. As a favor to the Shovelhead fans
that frequent the Nightrider Performance Site, the original text and drawings
are being presented. Changes used in the current Shovelhead engine build are
also noted.

With modification like this, it is quite possible for a
Shovelhead engine to run with the EVO and Twin Cam engines. We hope you
find the information helpful.

Area to be Modified

Original Modification Description

Current parts

Compression Ratio

9.5:1

9.5:1
Pistons by JE, Wiseco or Keith Black

Cylinder

Stock bore to be shortened. See figure 2.

3 5/8" bore standard length

Squish Band Clearance

.040" to .050". See figure 1

.040"

Piston

To be machined per figure 1 and figure 4.
Piston to cylinder clearance to be set at .002"